dumortierite

n.
A glassy pink, green, violet, or blue aluminum borosilicate mineral, Al₈BSi₃O₁₉OH, used in spark-plug ceramics and as imitation lapis lazuli.

[French, after Eugène Dumortier, 19th-century French paleontologist.]

A nesosilicate mineral with composition Al₇(BO)₃SiO₄)₃O₃. Dumortierite crystallizes in the orthorhombic system but well-formed crystals are rare; the mineral usually occurs in parallel or radiating fibrous aggregates. There is one direction of poor cleavage. The hardness is 7 on Mohs scale, and the specific gravity is 3.26–3.36. The mineral has a vitreous luster and a color that varies not only from one locality to another but in a single specimen. It may be pink, green, blue, or violet. Dumortierite is found in schists and gneisses and more rarely in pegmatites. In the United States it occurs at Dehesa, California, and at Rochester, Nevada, where it has been mined for the manufacture of high-grade porcelain. See also Silicate minerals.

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Al
Orthorhombic -- Rhombic bipyramidal

Environment

Scattered single crystals in pegmatites, in quartz concentrations in metamorphic rocks, and in gneisses and schists; also in solid, economically minable crystal masses.

Crystal description

Rarely in small embedded distinct crystals; usually in very finely fibrous compact masses.

Physical properties

Violet, pink-violet, or blue. Luster glassy to pearly hardness 7; specific gravity 3.3-3.4 (but usually impure); fracture conchoidal; cleavage poor pinacoidal. Translucent to transparent gemmy (very rare).

Composition

Alkaline aluminum borosilicate (64.6% Al ₂ O ₃ , 55% B ₂ O ₃ , 28.5% SiO ₂ , 14% H ₂ O).

Tests

On the charcoal under the blowpipe, it whitens, but with cooling, the color partially (or entirely) returns. Sometimes fluorescent blue after firing, sometimes naturally purple fluorescent.

Distinguishing characteristics

Bright color and fibrous appearance are distinctive, and distinguish it from nonfibrous-looking lazulite and lazurite (which usually have yellow to orange fluorescent associates). The great hardness distinguishes the purple variety from similar-appearing rare species or lepidolite.

Occurrence

In the U.S., dumortierite is most common in the West and has been mined in Oreana, Nevada, for spark plug ceramics. The blue is found in Los Angeles Co., California, with a gray quartz, and has been carved as an imitation lapis lazuli in China. Scattered needles through quartz are found in many localities and are recognized by their color. New York City building excavations produce fair dumortierite needles. Alpine, San Diego Co., California, dumortierite has a purple fluorescence. Blue "knots" in white quartz occur near Karibib, Namibia. A light "blue quartz" quartzite (comparable to green mica-saturated aventurine) from Brazil is colored by dumortierite. Several carat-sized gemstones, strongly dichroic in blue and white, have been found as waterworn Brazilian pebbles.

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